Thermal process for recovering oil

ABSTRACT

Disclosed herein is a thermal method for recovering oil from a subterranean formation in which a substantially cylindrical heated zone is created in the formation and in which heat can be introduced into the formation at a high rate. In the method a heated fluid (preferably steam) is injected down a well and into the formation at a pressure which is less than the formation breakdown pressure. Preferably, formation fluids are then withdrawn by means of the well. Subsequently, a heated fluid (again, preferably steam) is injected into the formation at a pressure greater than the formation breakdown pressure. Oil which has been heated by the injected fluids is recovered, preferably by means of the injection well, from the formation.

United States Patent 1 1 Woods et a1.

[ June 19, 1913 THERMAL PROCESS FOR RECOVERING OIL [75] Inventors:Edward G. Woods; Robert C. West,

both of Houston, Tex.

[73] Assignee: Esso Production Research Company,

Houston, Tex.

- [22] Filed: May 10, 1971 211 App]. Nos-141,908

[52] US. Cl. 166/303, 166/305 R [51 Int. Cl E21b 43/24 [58] Field ofSearch 166/303, 305 R, 308,

[56] References Cited 1 UNITED STATES PATENTS 3,292,702 12/1966 Boberg166/303 3,349,849 10/1967 2,939,688 6/1960 3,330,353 7/1967 3,459,2658/1969 Buxton 166/263 Primary Examiner-Robert L. Wolfe Att0rney-James A.Reilly, John B. Davidson, Lewis 11. Eatherton, James E. Gilchrist,Robert L. Graham and James E. Reed [57] ABSTRACT Disclosed herein is athermal method for recovering oil from a subterranean formation in whicha substantially cylindrical heated zone is created in the formation andin which heat can be introduced into the formation at a high rate. Inthe method a heated fluid (preferably steam) is injected down a well andinto the formation at a pressure which is less than the formationbreakdown pressure. Preferably, formation fluids are then withdrawn bymeans of the well. Subsequently, a heated fluid (again, preferablysteam) is injected into the formation at a pressure greater than theformation breakdown pressure. Oil which has been heated by the injectedfluids is recovered, preferably by means of the injection well, from theformation.

7 Claims, 4 Drawing Figures PAIENIEB m1 9 ms ll-I'l INVENTORS EDWARD G.WOODS ROBERT c. WEST BY ATTORNEY FIG.I

PATENIEU JUN r 9 I975 Sim-2N2 INVENTORS EDWARD e. wooos ROBERT 0. WESTAT TORNE Y THERMAL PROCESS FOR RECOVERING OIL BACKGROUND OF THEINVENTION 1. Field of the Invention This invention relates to therecovery of petroleum from a subterranean formation utilizing a well orwells for the injection of heated fluids and for the withdrawal ofpetroleum.

2. Description of the Prior Art Among the morepromising methods thathave been suggested or tried for the recovery of oil from viscous oilreservoirs are those which introduce thermal energy into the reservoirs.The viscosity of the oil in these reservoirs is generally so high thatthe oil cannot be recovered at economical rates using conventionaltechniques. However, the viscosity of these oils can generally beradically reduced by heating. Consequently, when thermal energy isintroduced into these reservoirs and the oil is heated, the viscositywill generally be reduced to a point that the oil can flow at efficientand economical rates.

The thermal energy may be in a variety of forms. Hot water, in situcombustion, and steam are examples of forms of thermal energy which havebeen used to recover oil from these viscous oil reservoirs. Each ofthese thermal energy agents can be useful under certain conditions.However, steam is generally the most efficient-and economical and isclearly the most widely employed thermal energy agent.

A number of suggestions have been advanced for improving the efficiencyof steaming operations" for oil recovery. Considerable effort has beendirected to one facet of this problem-increasing the fluid conductivityof such reservoirs. For example, it has been suggested that theformation may be fractured wth steam to increase its permeability and toplace heat quckly into tht formation at a substantial distance from thewellbore. Similarly, it has been suggested that, in an uncondolidatedformation, steam be injected at a pressure which is greater than theoverburden pressure to create a fluidized zone within the formation.Also, it has been suggested that a plugging agent be included in afracturing fluid to create an extensive fracture within the formationprior to steam injection. Each of these methods has the underlyingpurpose of increasing the permeability of the formation to the injectedheated fluid and of permitting the steam to travel a substantialdistance from the wellbore in a relatively short period of time.

These techniques can be effective in accomplishing their purpose ofcreating a highly conductive flow path within the formation for injectedsteam and of heating the formation at a substantial distance from thewell. These fracture systems are plane-like heat sources and can bebeneficial 'to certain oil recovery processes. For example, where steamis continuously injected through one well into a fracture intheformation to drive oil before it to an offset producing well, the heatfrom the fracture covers a wide portion of the formation. Nevertheless,such planar heat sources can be undesirable in other steamingoperations.

In steam stimulation processes, it is preferable to retain the heat nearthe injection well. In this process, commonly referred to as thehuff-and-puf process, steam is injected into the formation through awell and subsequently heated oil is withdrawn from the formation bymeans of the same well. Since the same well is used for both injectionand production, it is desirable to have a substantially cylindricalheated zone around the well. Such a cylindrical heated zone is mostefficient in transferring thermal energy to the oil in the formation. Afracture formed in accordance with the teaching of the prior art at thelocation of the injectionproduction well will not assist in forming acylindrical heated zone; the heated zone around such a fracture wouldresemble an ellipse with a high degree of eccentricity. The thermalenergy which passes through the fracture heats oil in the more remoteareas of the reservoir. This heated oil which is remote from the wellhas a tendency to cool to the point where it will no longer flow beforeit can be produced. As a consequence, the efficiency of the processdeclines.

SUMMARY OF THE INVENTION In the practice of this invention, the heatedzone around an injection well has a desirable configuration, i.e.,approximately cylindrical, and the zone has a high conductivity toinjected fluids. These desirable results are accomplished by firstheating the formation by injecting a heated fluid, such as steam, at apressure which is less than the formation breakdown pressure. This firstinjection step creates a heated zone which is substantially cylindrical.Preferably, fluids are then withdrawn from the formation by means of thewell to reduce the oil saturation within the heated region.Subsequently, a heated fluid, which also may be steam, is injected intothe formation at a pressure exceeding th formation breakdown pressure.This second injectir .1 step generally creates a fracture orpressure-parting of the formation which is highly conductive to injectedfluids. In addition, due to the higher pressure of the second injectionstep a relatively high fluid injection rate is achieved. However, due tothe first injection step, the fracture is less extensive than it wouldbe in the absence of the first injection step. As a consequence, theheated region around the injection well is larger but remainsapproximately cylindrical. Thus, in the practice of this invention, twodesirable results are achieved. The heated fluid can be injected at ahigh rate, and the heated region around the injection well retains anapproximately cylindrical configuration.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic elevation view inpartial section of a well intersecting an oil-bearing formation.

FIG. 2 is a schematic plan view of the oil-bearing formation penetratedby the well and illustrating the heated regions around the wellfollowing the initial injection of steam into the formation.

FIG. 3 is a schematic plan view of the oil-bearing formation penetratedby the well and illustrating the heated regions around the wellfollowing the subsequent injection of steam.

FIG. 4 is a schematic plan view of an oil-bearing formation penetratedby a well and showing the heated regions around the well where theformation is fractured in accordance with the teachings of the priorart.

DESCRIPTION OF THE INVENTION The practice of this invention can perhapsbe most easily understood by reference to the drawings. Referring toFIG. 1, an oil-bearing formation shown generally at 10 is penetrated bya well or borehole 11 which has been drilled from the surface of theearth (not shown). A string of large diameter pipe or casing 12 isplaced in the hole and bonded to the walls of the well by the cementsheath 13 in a conventional manner. The casing 12, the cement sheath 13,and the formation are then perforated to provide paths 14 for fluidcommunication between the interior of the casing and the formation. Astring of small diameter pipe or tubing 15 is suspended within thecasing 12 for the injection of fluids into, and for the withdrawal offluids from, the formation. The exterior of the tubing is fitted with apacker assembly 16 which engages the interior of the casing to bar fluidcommunication within the casingtubing annulus at that location. Itshould be understood that this is a conventional well completion whichmay be used in the practice of this invention. However, the invention isnot limited to this specific apparatus. Any well completion apparatuswhich is capable of being used in the following described process willbe satisfactory.

With a completion assembly such as that illustrated in FIG. 1, a heatedfluid is injected down the tubing and into the oil-bearing formation. Aswill be discussed in more detail later, a number of fluids may be usedin the practice of this invention. However, saturated steam generallywill be preferred and the most convenient to use. The invention willtherefore be discussed in terms of saturated steam.

The initial volume of saturated steam is injected at a pressure which isless than the fracturing pressure for the formation. As a consequence ofthis initial steam injection, steam will exist out to some radial point17 from the well. Hot water from condensed steam and heated formationwater will extend for a further distance into the reservoir to a radialpoint 18.

The configurations of the heated zones within the formation followingthe injection of the initial volume of saturated steam can perhaps bemore clearly seen in FIG. 2. As shown in FIG. 2, the portion of theformation which has been heated to steam temperature is substantiallycircular in cross-section and cylindrical in volume. The portion of theformation containing the hot water from condensed and heated connatewater 18 will form an annular ring around the steam heated zone 17. Itwill be understood, of course, that the configurations of the heatedzones shown in FIG. 2 and in subsequent FIGURES are illustrative of theprocess only. These precise geometric shapes would not be realized inmost instances due to the presence of permeability streaks, faults orother reservoir heterogeneities that would more or less distort theheated zones from the configurations illustrated. However, the benefitsof this method will still be realized even though such reservoirheterogeneities are present. Where the method of this invention is used,heat can be introduced into the reservoir at a relatively high rate andthe heated regions around the well will be more nearly cylindrical thanthey otherwise would have been.

After the injection of the initial volume of steam, it will generally bepreferred to shut the well in and to permit the formation to heat-soak.During this heatsoaking period, thermal energy is transferred from theheated regions 17 and 18 to the rock matrix and to the formation fluids,including oil. This heating of theoil reduces its viscosity and makes itmore susceptible to flow.

Preferably, the ziwellis then opened to production, and formation fluidsincluding the heated oil are withdrawn by means of the tubing 15. Thisintermediate production step will reduce the oil saturation within theheated regions, thus, increasing the conductivity of these regions tosubsequently injected steam.

Next, a relatively large volume of saturated steam is injected throughthe tubing 15 and into'th'e formation 10 at a pressure in excess of thatrequired to fracture or pressure-part the formation. In most instances,this high pressure will, in fact, create a fracture 19 as shown in FIG.3. However, due to the initial steam injection step, the resultantfracture will be less extensive than it otherwise would have been. Theinitially injected steam will reduce the viscosity of the formation oilwithin the heated region and will increase the water saturation in theimmediate vicinity of the well. As a consequence, when the steam isinjected at a high pressure as a fracturing fluid it will have atendency to rapidly bleed or leak into the heated region and less steamis available to pressure-part or fracture the formation. This bleedingor leak-off will be even more pronounced where oil has been withdrawnfrom the formation prior to the injection of the high pressure steam.This intermediate production step will have the tendency of furtherreducing the oil saturation within the heated regions 17 and 18 andconsequently increasing the water saturation within these regions.Naturally, the high water saturation within the heated regions 17 and 18will further promote bleed-off of the high pressure steam.

The results of these steps can be seen diagramaticall in FIG. 3. Theportion of the reservoir that has been heated to steam temperature 17has been greatly expanded due to the injection of the large volume, highpressure step. However, the heated regions have maintained asubstantially cylindrical configuration due to thev relatively smallfracture which was formed.

The advantages of this invention over the prior art methods can perhapsbe more clearly understood by comparison to FIGS. 2 and 4. FIG. 2 (inaddition to illustrating the heated regions in the reservoir followingthe initial steam injection step) would be representative of the heatedregions existing within a formation in those prior art methods where thesteam injection pressure was maintained below the point where theformation would fracture or pressure-part. Although the heated regionsshown in FIG. 2 have a substantially cylindrical configuration, they aremuch less extensive than the heated regions formed in accordance withthe teaching of this invention as illustrated in FIG. 3. FIG. 4 isillustrative of the heated regions within a formation created in thoseprior art methods where high pressure steam is injected into theformation without the preceding low pressure steam injection step. Inthis instance, the fracture 19 extends for a considerable distance intothe formation. However, the heated regions 17 and 18 do not have themore desirable cylindrical configuration; these heated regions morenearly resemble an ellipse with a high degree of eccentricity. The oilwhich exists near the tips of the extended fracture 19 is initiallyheated to the point that it is capable of flow, but due to its distancefrom the well, it has a tendency to cool to the point where it will nolonger flow before it can be produced.

As'was previously stated, the heated fluid which is preferred for use inthe initial and subsequent injection sequences is saturated steam. Steamgeneration units which will produce saturated steam at the pressure,temperature and quantity required for the practice of this invention arereadily and commercially available. The steam produced by such unitsgenerally has a quality of from about 60 to 90 per cent.

The heated fluid may also be hot water or superheated steam. However,these fluids are generally not preferred. Hot water is less efficientthan steam in transferring thermal energy to the oil since steam canrelease its latent heat of vaporization as well as its sensible heat.Superheated steam requires the use of expensive surface equipment suchas a water knockout column downstream from the steam generator orexpensive multi-pass steam generation equipment.

Although the heated fluid in the initial and subsequent injectionsequences are preferably the same, i.e., saturated steam, these fluidsmay differ. For example, the initially injected fluid may be steam andthe second injected fluid may be hot water or vice versa. As a furtherexample, the initial fluid may be hot water and the subsequent fluidsuperheated steam.

With reference to the initial steam injection, it was previously statedthat steam would be injected at a low pressure and low volume. Thepressure employed should be less than the formation breakdown orfracture pressure; the pressure necessary to fracture or pressure-part aformation will be discussed in greater detail hereinafter. At thispoint, it is only necessary to note that during the initial injectionstep, the pressure of the steam should be below this level.

The quantity of steam employed during the initial steam injection steppreferably should be sufficient to heat the formation to steamtemperature for a distance from about to about 100 wellbore radii fromthe well. One purpose of this initial steam injection step is to heatthe oil in the formation and, thus, reduce its viscosity and to enableit to flow. The flow rate of the oil in a radial system is dependentupon the pressure differential existing between the formation and thewell. Furthermore, most of the pressure drop in a flowing radial systemwill occur very near the wellbore due to the logarithmic variation ofpressure differential with drainage radius. For example, it has beencalculated that approximately per cent of the pressure drop occurswithin 25 wellbore radii from the well; approximately 60 per cent ofthe-pressure drop occurs within 100 wellbore radii. Muskat, PhysicalPrinciples of Oil Production, 1949, McGraw-Hill Book Co. Inc., New York,N.Y. Methodsfor determining the quantity of steam which must be injectedto heat the formation and oil to steam temperatures to these distancesare well known to those skilled in the art. See, for example, Farouq,Ali, Marx and Langenheims Model of Steam Injection, Producer's Monthly,Nov. l966, pp. 2-8.

As was previously stated, the subsequently injected steam is injected ata high pressure and in a large volume. The injection pressure should beat least as great as the formation breakdown or fracture pressure. Aformation is normally fractured by injecting fluid down the well casingor tubing at rates higher than the rock matrix will accept. This rapidinjection produces a build-up in wellbore pressure until a pressurelarge enough to overcome compressive stresses within the formation andthe tensile stress of the rock matrix is reached. At this pressure,formation failure occurs and a fracture or pressure-part is generatedwithin the formation. The pressure at which a formation will fracture isdependent upon a number of variables including the tensile strength ofthe rock, the rate at which the fracturing fluid will bleed into theformation, the extent to which the oil contributes to the competence ofthe formation and the like. For a given formation at a given location,the effect of these variables and the pressure necessary to breakdown orfracture the formation is generally well known. In some instances wherea field experience is not extensive it may be necessary to estimate theformation breakdown pressure by taking these variables intoconsideration by methods which are well known to those skilled in theart. It is even possible to roughly estimate the formation breakdownpressure by calculating the overburden pressure existing at theformation. It is generally considered that a pressure equal to fromabout 0.6 to about 1.0 times the overburden pressure will create afracture.

Due to the high steam bleed-off created by the preheating of theformation no fracture may, in fact, be formed during this subsequentsteam injection step. There may be a pressure-parting of the formationor a fluidized zone which forms a channel of high fluid conductivity.However, in extreme circumstances, even such a fluidized zone may not becreated due to the extreme bleed-off of steam during the high pressureinjection. Under these circumstances, the heated region around thewellbore will be even more nearly cylindrical than that which would beformed in the presence of a fracture. This, of course, would bedesirable since the heated oil would have the shortest possible flowpath to the well.

The quantity of steam employed in this high pressure, large volume stepwill generally be from about five to about 20 times greater on a weightbasis than that injected during the initial steam injection step. Thisvol ume will, of course, be dependent on reservoir conditions andexisting facilities.

Following the injection of the high pressure, high volume steam theformation will again be shut in and permitted to heat-soak. Followingthis heat-soaking, the well will be returned to production. When theproduction from the well declines the formation can, of course, berestimulated by subsequent injection of additional quantities of steam.

The principle of the invention and the best mode in which it iscontemplated to apply that principle have been described. It is to beunderstood that the foregoing is illustrative only and that other meansand techniques can be employed without departing from the true scope ofthe invention as defined in the following claims.

What is claimed is:

l. A method of recovering oil from a subterranean formation whichcomprises injecting a heated fluid into the formation by means of a wellat a pressure less than the breakdown pressure, then withdrawing oilfrom the formation by means of the well, subsequently injecting a heatedfluid into the formation by means of the well at a pressure greater thanthe formation breakdown pressure, and recovering oil from the formationby means of the well.

2. A method as defined in claim 1 wherein the first injected heatedfluid is steam.

3. A method as defined in claim 2 wherein the steam is injected insufficient quantity to heat the formation to steam temperature at adistance of at least 25 wellbore radii from the well.

as great on a weight basis as the quantity of the first injected heatedfluid.

7. A method as defined in claim 1 further comprising injecting furtherquantities of heated fluid into the formation subsequent to recoveringoil from the formation and producing further quantities of oil from theformation.

2. A method as defined in claim 1 wherein the first injected heatedfluid is steam.
 3. A method as defined in claim 2 wherein the steam isinjected in sufficient quantity to heat the formation to steamtemperature at a distance of at least 25 wellbore radii from the well.4. A method as defined by claim 3 wherein the steam is injected in aquantity to heat the formation to steam temperature at a distance of nomore than 100 wellbore radii from the well.
 5. A method as defined byclaim 1 wherein the second injected heated fluid is steam.
 6. A methodas defined by claim 5 wherein the quantity of the steam is from aboutfive to about 20 times as great on a weight basis as the quantity of thefirst injected heated fluid.
 7. A method as defined in claim 1 furthercomprising injecting further quantities of heated fluid into theformation subsequent to recovering oil from the formation and producingfurther quantities of oil from the formation.